New color palettes can be added at the \Documents\My Tableau Repository\Preferences.tps file.
You can add new palettes manually by editing the file, or replace it with another tps that someone build.
Note to change the name back to Preferences.tps.
An excellent list of palettes is from the famous Color Brewer. Someone has created it for Tableau, which can be downloaded here.
New shapes/symbols can be added to Tableau by creating a new folder or adding into existing folders within
Documents\My Tableau Repository\Shapes.